This invention relates to a psychrometer for measuring the humidity (a term which is used herein to mean the degree of saturation with any desired vapor, not merely water vapor) of a gas flow. The instant psychrometer is especially useful for measuring the humidity of gas flows at temperatures of about 93.degree. to about 260.degree. C.
Numerous forms of psychrometer are known in the prior art. The most relevant prior art of which the applicants are aware may be summarized as follows.
U.S. Pat. No. 1,603,243 to Irwin describes an early form of psychrometer in which the incoming air is heated by means of an electric heater. The psychrometer is intended for use at substantially ambient temperatures, the purpose of the heating being to prevent the formation of ice around the wet bulb.
U.S. Pat. No. 1,942,934 to Reeve describes a psychrometer in which a wick passes through a constricted opening so that the main bulk of the water supply to the wick is isolated from the gas stream passing through the psychrometer.
U.S. Pat. No. 2,107,077 to Magner describes a psychrometer in which the wet bulb has a wick surrounded by a sock so that the gas the humidity of which is being measured passes over the sock and keeps the water supplied to the wet bulb at substantially the same temperature as the gas stream. This form of psychrometer is, however, capable of operating only at relatively low temperatures.
U.S. Pat. No. 2,623,391 to Malecki describes a psychrometer in which the wet bulb is surrounded by a mass of porous material to improve thermal conductivity to the wet bulb and thus to secure better sensitivity and quicker response to vapor concentration changes.
U.S. Pat. No. 3,196,683 to Gross describes a psychrometer in which, to prevent excessive evaporation of water from the wick and consequent fouling thereof at the point where the wick is exposed to the air flow, the wick is enclosed within a tube which is split so as to allow only an extremely small area of contact between the wick and the air flow.
U.S. Pat. No. 3,459,034 to Kawaguchi describes a psychrometer for measuring the moisture content of a gas at high temperatures in which a sample of gas is admitted to a measuring chamber and a wet bulb within the chamber is supplied with water from a reservoir which is kept cooled below the boiling point of water.
U.S. Pat. No. 3,515,001 to Greenspan, et al. describes a psychrometer for measuring the humidity of a stream of gas containing vapor of a condensible liquid in which a sample stream of the vapor/gas mixture is flowed over the wick and a heat exchanger, while a stream of liquid corresponding to the condensed vapor of the vapor/gas mixture is counterflowed through the heat exchanger to the wick.
U.S. Pat. No. 3,603,135 to Kawaguchi describes a high-temperature psychrometer in which the wet bulb is detachably inserted into a sleeve made of a temperature-resistant capillary material, the base portion of this sleeve being exposed to the passage of water in a predetermined amount and at a predetermined pressure so that the sleeve is maintained in a reproducably wet condition.
U.S. Pat. No. 3,886,797 to Bauer describes an electrical-resistance psychrometer provided with means for uniform supply of an evaporating liquid from a supply tank to an evaporator body made of a porous material which surrounds the "wet" resistor, this wet resistor having an appreciable electric current passed therethrough so as to increase its temperature.
U.S. Pat. No. 4,129,250 to Chaikin, et al. describes a psychrometer intended for measuring the humidity of exhaust air from industrial driers (and thus capable of operating at moderately high temperatures) in which a wet bulb thermocouple is physically dipped into a bowl of water under the control of a timer system to control the amount of exhaust air discharged until the wet bulb thermocouple is equilibrated with the exhaust air sampled after dipping.
U.S. Pat. No. 4,222,261 issued Sept. 16, 1980 to Leblanc, et al. describes a high-temperature psychrometer in which the wet bulb is surrounded by a sheath of porous material, which is in turn surrounded by a screen to prevent thermal radiation evaporating too much liquid from the sheath. A dosing pump is provided for injecting predetermined amounts of volatile liquid at regular intervals into the interior of the sheath and onto the screens to ensure sufficient moistening of the sheath and the screens.
Zagorzycki, "Automatic Control of Conveyor Driers", Chemical Engineering Practice 75(4), 50 (1979) discusses the drying of food products and the limitations of prior art psychrometers.
Those skilled in the art are aware that dry bulb-wet bulb psychrometers, such as those discussed above, wherein a humid gas stream is passed over a continuously wetted wick and measurements of gas flow temperature (hereinafter referred to as the dry bulb temperature of the gas flow) and wetted wick temperature (hereinafter referred to as the wet bulb temperature of the gas flow) are used to determine humidity, can be used to measure humidity at dry bulb temperatures up to about 121.degree. C., but special precautions are necessary and results are of questionable accuracy above 100.degree. C. The most difficult problem at temperatures above 100.degree. C. is keeping the wick saturated with water and in previous psychrometers intended for use above 100.degree. C., the velocity of the gas flow had to be above about 4.5 m/sec. Also, heat conduction and heat radiation have led to erroneous readings at the higher temperatures.
Dew point cells, using hygroscopic salts such as lithium chloride, are rugged, easy to maintain and can operate up to dry bulb temperatures of about 104.degree. C., but the dew point temperature must not exceed about 80.degree. C. Electronic hygrometers, which change impedence or capacitance with changes in humidity, and optical dew point cells, which use a thermoelectrically-cooled mirror to sense that temperature at which water is in equilibrium with the vapor in the air can only be used up to dry bulb temperatures of about 94.degree. C.
Thus, no really satisfactory method exits for measuring humidity of gas streams having dry bulb temperatures of about 94.degree. to about 260.degree. C. and wet bulb temperatures of about 38.degree. to about 99.degree. C., despite the fact that evaluation of energy content, corrosive properties and chemical composition of air streams necessitate quantitative measurements of the humidity of gas streams within these ranges. The instant invention seeks to provide a psychrometer which is useful within these temperature ranges.